A telephone communications system is an on-site call processing system that interconnects a plurality of on-site telephone station sets to other on-site telephone station sets and to a plurality of lines connected to a central office or a private branch exchange. This call processing system generally operates under stored program control and provides a plurality of features to the user, including call forwarding, ability to hold a call, ability to add or drop lines from a conference call and in addition, the basic feature of allowing each station set user to originate and receive station-to-station calls.
To permit full use of these and many additional features of the communications system, the individual station set must provide the user with control access and with information as to its present operational status. This is normally accomplished by use of a multibutton electronic telephone station set having a plurality of push buttons which are used to select modes of operation and a plurality of visual indicator devices, associated therewith, serving to define the present operational status and the line selection.
These visual indicator devices and a DC-to-DC converter included in each station set are normally energized by DC electrical power supplied through the station set telephone line, and generally comprise two LED devices associated with each of the various control buttons of the station set. They are continuously active and consume power of approximately two watts with at least one LED device on, even though the station set is inactive or out of use for a considerable length of time. The most significant inactive period comprises overnight and weekend intervals during which a communications system in a typical business office is often inactive and unused.
In an era of rising energy costs, the dissipation of energy to continuously maintain visual indicator devices active and power inactive station sets during extended intervals of nonuse represents a considerable cost to the subscriber and yet, to deactivate the communications system during off hours to save energy is unacceptable since all stations are denied any off hours use. Given the energy cost reduction desirability and the need of providing continuous normal service to each station set, a practical energy saving control scheme for a communications system must have considerable flexibility to permit full station set service at all times.
A need for every station set in a telephone communication system to be simultaneously powered is an extremely rare event. Accordingly, the power level selected to be supplied may be chosen on the assumption that only a certain percentage of station sets will be used at any one time. However, in those instances where service demand exceeds that level, either the power source is overloaded and a substandard level of power is supplied to each station set or station sets are left to appear dead or malfunctioning with no indication that service is being temporarily denied.